1. Field of the Invention
This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature measuring devices and analog-to-digital converters in integrated circuit systems.
2. Description of the Related Art
Many digital systems, especially those that include high-performance, high-speed circuits, are prone to operational variances due to temperature effects. Devices that monitor temperature and voltage are often included as part of such systems in order to maintain the integrity of the system components. Personal computers (PC), signal processors and high-speed graphics adapters, among others, typically benefit from such temperature monitoring circuits. For example, a central processor unit (CPU) that typically “runs hot” as its operating temperature reaches high levels may require a temperature sensor in the PC to insure that it doesn't malfunction or break due to thermal problems.
Often, integrated circuit (IC) solutions designed to measure temperature in a system will monitor the voltage across a diode (or multiple diodes) at different current densities to extract a temperature value. This method generally involves amplifying (or gaining up) a small voltage generated on the diode(s), and then subtracting voltage from the amplified temperature-dependent voltage in order to center the amplified (gained) value for conversion by an analog-to-digital converter (ADC). In other words, temperature-to-digital conversion for IC-based temperature measuring solutions is often accomplished by measuring a difference in voltage across the terminals of typically identical diodes when different current densities are forced through the PN junctions of the diodes. The resulting change in the base-emitter voltage between the diodes (ΔVBE) is generally proportional to temperature. More specifically, a relationship between the base-emitter voltage (VBE) and temperature is defined by the equation       V    BE    =            kT      q        ⁢    ln    ⁢          I              I        s            where k is constant, q represents charge, T represents absolute temperature, ls represents saturation current and I represents the collector current. A more efficient and precise method of obtaining ΔVBE is to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔVBE may be related to temperature by the equation       Δ    ⁢                   ⁢          V      BE        =            kT      q        ⁢          ln      ⁡              (        N        )            where N is a constant representing a preselected ratio of the two separate currents that are supplied to the PN junction of the diode.
A typical dynamic range of ΔVBE, however, is small relative to dynamic ranges that are typical of analog-to-digital converters (ADCs). That is, ΔVBE, which is used to measure the PN junction temperature, generally has a small dynamic range, for example on the order of around 60 mV in some systems. Therefore it is generally required to further process ΔVBE in order to match the dynamic range of ADCs. Typically, in order to obtain the desired conversion values at various temperatures, ΔVBE is multiplied by a large gain, and then centered to zero, which can be accomplished by subtracting a fixed voltage.
In general, implementations today perform the temperature signal processing (TSP) in a separate temperature sensor circuit that generates a sufficiently large voltage signal, which is fed into a separate ADC that may have been designed using a number of different topologies. Temperature-to-digital converters (TDC) of such implementations usually contain complex circuits with high power dissipation. The yield of these TDCs during the fabrication process may also be low as there are many components that need to be matched for a given process spread.
An example of a typical temperature measurement system, which includes an ADC, is illustrated in FIG. 1. A TSP circuit 100 is coupled to an ADC 130. TSP 100 may comprise current sources 104 and 106, where a current provided by 104 is an integer (N) multiple of a current provided by 106, a diode 102, an integration capacitor 126, an offset capacitor 122, a gain capacitor 124, and an operational amplifier (OP-AMP) 120, interconnected as illustrated in FIG. 1. P1110 and P2112 represent non-overlapping clocks that provide switching between two circuit configurations as shown. When P1110 is closed, current source 104 powers TSP 100 and P2112 is open. Similarly, when P2112 is closed, current source 106 powers TSP 100 and P1110 is open. Switching between current sources 104 and 106, different currents are forced through the junction of diode 102 resulting in a change in diode-junction-voltage (ΔVBE). Although omitted in FIG. 1, it should be understood that when either P1110 or P2112 is open, the respective uncoupled current source may be shunted to ground. In the circuit configuration shown, voltage sampling occurs when P1110 is closed, and charge transfer takes place when P2112 is closed. In other words, during operation, switching from a configuration of P1110 closed and P2112 open to a configuration of P1110 open and P2112 closed, results in ΔVBE effectively “pumping” charge to gain capacitor 124, which in turn leads to integration capacitor 126 also receiving a charge. More specifically, opening P1110 and closing P2112 results in a value drop of diode-junction-voltage VBE, expressed as ΔVBE. Consequently, ΔVBE appears across the terminals of capacitor 126, in case capacitor 126 is equal in value to capacitor 124. If capacitor 124 is greater in value than capacitor 126, then ΔVBE will be amplified, or “gained up”, hence an amplified value Vtemp 130 will appear at the output of OP-AMP 120. Voffset 132 is subtracted through offset capacitor 122.
Voltage-temperature relationships characterizing TSP 100 may be described by the following equations:Vtemp=CI/CT*ΔVBE(T)−CI/CO* Voffset, whereCI/CT=(ADC dynamic range)/(ΔVBE (Tmax)−ΔVBE(Tmin)), andVoffset=(CI/CT*ΔVBE(Tmax)−(ADC dynamic range))*CO/C1.Tmax and Tmin represent maximum and minimum diode temperatures, respectively. ADC dynamic range indicates a range of valid voltage values required for proper ADC operation. Disadvantages of the typical system as illustrated in FIG. 1 include a need for large capacitors (such as CI and CT) to meet matching requirements for a fixed-gain amplifier. Also, in order to perform a fixed-gain function, an additional amplifier is required in addition to amplifiers required to perform the ADC function.
Therefore, there exists a need for a system and method for designing a more accurate and less area-intensive temperature-to-digital converter with a reduced number of capacitor components and amplifiers.